Field of the Invention
The present disclosure relates to an electrophotographic image forming apparatus. In particular, the present disclosure relates to technology for controlling light amount of laser light used for image formation.
Description of the Related Art
The electrophotographic image forming apparatus performs image formation by irradiating a photoreceptor with laser light to form an electrostatic latent image and adhering developer to the electrostatic latent image. The laser light is output from an optical scanning apparatus comprising an exposure unit comprising a semiconductor laser, a rotating polygon mirror, f-θ lens etc. The semiconductor laser outputs the laser light. The laser light output from the semiconductor laser is periodically deflected by the rotating polygon mirror as the laser light and irradiates the photoreceptor through the f-θ lens. The laser light scans the photoreceptor according to the rotation of the rotating polygon mirror.
The optical scanning apparatus comprises a detection sensor for detecting the laser light (hereinafter, referred to as “BD (Beam Detector)”. The BD receives the laser light before or after the laser light scans the photoreceptor. By receiving the laser light, the BD generates BD signals. The BD signal is used to control emitting timing of the laser light based on an image data in one scanning cycle. Further, the BD signal is used to define switching timing of control mode of a laser driver. United States Patent Application Publication No. 2005/212901 discloses an image forming apparatus, in which light amount of the laser light made incident on the BD to generate the BD signal becomes equal to that of the laser light which scans the photoreceptor.
The image forming apparatus disclosed in the United States Patent Application Publication No. 2005/212901, however, has following problems. FIG. 8 is a timing chart of light emission control of the semiconductor laser used for a conventional optical scanning apparatus. As shown in FIG. 8, there may be a case where the light amount of the laser light which scans the photoreceptor needs to be controlled to cope with a state change of the image forming apparatus main body when continuously forming images or to cope with a rapid environmental change of the image forming apparatus. At this time, as shown in FIG. 8, by changing a value of reference voltage Vref, the light amount of the laser light which scans the photoreceptor is controlled. Like the conventional image forming apparatus, if the light amount of the laser light made incident on the BD is equally set to the light amount of the laser light which scans the photoreceptor, the waveform of the BD signal varies before and after the control of the light amount. This causes a problem that a writing start position of the image differs before and after the control of the light amount. To solve this problem, an image forming apparatus capable of separately controlling the light amount of the laser light for generating the BD signal and the light amount of the laser light which scans the photoreceptor is desired.